1. Field of the Invention
The present invention relates generally to modulation and power control in a wireless communication device transmitter, and more particularly to a system for implementing amplitude and phase modulation with a closed or open power control feedback loop capable of supporting either a linear or a non-linear saturated mode power amplifier.
2. Description of the Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone like a communication handset. These variations include, for example, global system for mobile communication (GSM) radios, code division multiple access (CDMA) radios, integrated dispatch enhanced network (IDEN) radios and terrestrial trunked (TETRA) radios. Generally, each of these different types of radios requires a different modulation and power control method to meet system specifications The different modulation and power control schemes each have advantages and disadvantages.
In a typical GSM system, a gaussian minimum shift keying (GMSK) modulation scheme supplies a very clean, phase modulated (PM) or frequency modulated (FM) transmit signal to a non-linear power amplifier (PA). A non-linear saturated PA is ideally suited for GMSK modulation because the input RF waveform exhibits information only in its phase component. In such an arrangement, the non-linear PA is highly efficient at modulation of the phase signal and minimizing power consumption. Because the modulated signal is typically supplied directly from a GMSK modulated phase lock loop (PLL), the need for filtering, either before or after the PA, is also minimized.
Other transmission standards are now being developed in which both a PM signal and an amplitude modulated (AM) signal are transmitted. Standards such as these often increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, existing GSM modulation schemes and chip set architectures are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a signal containing PM and AM components, a highly linear PA is typically required. Unless the PA is very linear, it may distort the combined transmission signal by causing undesirable AM to AM and AM to PM distortion. Linear PAs are very inefficient, however, thus consuming significantly more power than a non-linear PA and drastically reducing the life of the battery or other power source.
Modern wireless communication devices typically contain a multi-mode transmitter that can function as multiple types of radio communication devices effective to generate multiple transmission mode signals from the input baseband signal. For example, one radio communication device may function as both a GSM and an EDGE (Enhanced Data GSM Evolution) radio communication device. The EDGE system requires a dual communication function in which communication of lower rate voice signals is performed on GMSK modulation (AM), while higher rate data communication is typically performed in π/8-rotating 8-PSK (Phase Shift Keying) modulation (AM and PM). The 8-PSK modulation format (also referred to as the EDGE-mode) can be regarded as a modulation in which an amplitude shift is further added to a phase shift of a carrier signal in the GMSK modulation. While 1-bit information is sent per symbol in the GMSK modulation format, 3-bit information is sent per symbol in the 8-PSK modulation format. This provides an advantage that communication in the EDGE system can be performed at a higher transmission rate than in the GSM system, while maintaining the same transmission bandwidth as the GSM system. However, the power consumption benefits of using a non-linear, highly efficient PA when employing a GMSK system are not realized in the EDGE system using a highly linear PA. To address this deficiency, numerous architectures have been proposed to implement both GMSK and EDGE using non-linear, more efficient PAs
One dual GSM/EDGE system found in radio communication devices is the “full-polar” modulation architecture, where a polar modulator applies amplitude modulation to the supply or bias control port of a non-linear PA while simultaneously applying a phase modulated signal to the PA's RF input port. In the GMSK mode, the modulator only supplies a phase modulated signal to the RF input to the PA. In a full-polar modulation architecture, the amplitude modulation is applied to the PA by controlling the bias current, the collector voltage, or a combination of both via an analog voltage-control input to the PA. Because the PA is non-linear, a power control loop is often employed with the AM modulation applied to one or more ports of the power control loop. Therefore, the power control loop provides both a power control signal and an AM signal to the PA.
A second dual GSM/EDGE system is the “polar-lite” modulation architecture, where the polar modulator applies amplitude and phase modulation to an RF stage prior to a PA in EDGE mode. The PA stage that follows the RF stage must operate in the linear mode because the input signal contains both AM and PM components. In GMSK mode, the PA bias is reconfigured to operate in the more efficient saturated non-linear mode.
Modulation formats and architectures are continuously updated to reflect new approaches to maximize information transfers in limited bandwidths. Changes in standards or standards based on newly available spectrum may also cause designers to approach modulating transmitters with different techniques. Moreover, a preference for full-polar or polar-lite has not materialized in the market. As a result, wireless mobile handset manufacturers may manufacture an array of handset types implementing varied modulation schemes and standards. Further, manufacturers desire to quickly modify transmitter architectures as market and standards-based changes influence what is a preferred implementation, but this can be difficult when different architectures are designed with different components. For example, full-polar modulation architectures use a saturated PA for both GMSK and EDGE transmissions, while polar-lite modulation architectures require a dual-mode PA, which operates in a non-linear mode for GMSK transmissions and a linear mode for EDGE transmissions. Linear mode and dual-mode PAs continue to be widely manufactured for GSM and EDGE handsets. There are cost, size, performance, and manufacturability tradeoffs associated with linear and dual mode PAs.
As can be seen from all of the above problems, it would be desirable to implement a common transmitter architecture configurable to support multiple modulation schemes and physical components. For example, given the current market acceptance, it would be desirable to provide a transmitter platform configurable to support both polar-lite and full-polar transmitter modulation.